Fast-annealing malleable cast iron method

ABSTRACT

METHOD OF MANUFACTURING PEARLITIC AND/OR FERRITIC MALLEABLE CASTINGS, WHEREIN THE WHITE-CAST PIECES ARE ANNEALED AT A TEMPERATURE AND DURING A TIME SUFFICIENT TO DECOMPOSE THE CEMENTITE, THIS METHOD BEING CHARACTERISED BY THE USE OF A CAST IRON CONTAINING AT LEAST 2% OF SI, AND BY THE FACT THAT BEFORE THE CASTING FROM 0.1% TO 0.3% OF MISCHMETALL IS ADDED, THE SULFUR CONTENT BEING KEPT AT S$0.025% TO PERMIT THE REDUCTION OF THE TOTAL ANNEALING HEAT-TREATMENT CYCLE TO LESS THAN 10 HOURS.

United States Patent Ofice 3,565,698 Patented Feb. 23, 1971 3,565,698 FAST-ANNEALING MALLEABLE CAST IRON METHOD Christian de Mercoyrol de Beaulieu, Billancourt, France,

assignor to Regie Nationale des Usines Renault, Billancourt, France, and Automobiles Peugeot, Paris, France No Drawing. Filed Mar. 7, 1968, Ser. No. 711,210

Claims priority, application France, Apr. 5, 1967,

Int. C1. 0213 5/06, 9/00 US. Cl. 1483 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of manufacturing ferritic and/or pearlitic malleable cast iron by using a short annealing time permitted by an in rement in the silicon content.

Malleable cast iron is currently used in mechanical industries, notably in the construction of automobiles, for making miscellaneous castings. The rough-cast parts have a whitish structure comprising cementite and ferrite or pearlite, and must be free of graphite. In this condition, the castings are fragile and hardly machinable. A prolonged annealing at high temperature will decompose the cementite into ferrite and graphite, and a suitable heattreatment cycle will provide the desired structure.

This annealing step is absolutely necessary although it takes a relatively long time; therefore, it is customary, in the case of a cast iron manufactured according to the cupola-furnace and reverberatory furnace duplex process, to carry out a first annealing step of 15 hours at 950 C. followed by 24-hour continuous heating step at 745 C. which, considering the time periods necessary for attaining the desired temperature and subsequently cooling the cast iron, involves processing times of 48 to 72 hours. In the case of modern installations wherein the cast iron is melted in high-frequency furnaces and annealed in thrust furnaces, the resulting cycles still take from 30 to 40 hours.

The thus processed malleable irons approximate the following composition: C=2.5%; Si=l.3%; Mn=0.5%; S 0.l0%; P 0.l0%; Fe=the balance.

The function of silicon is particularly important and the rapidity of the annealing step is subordinate to its content. If the silicon content increases, the annealing time decreases, but there is a risk of developing primary graphite in the white structure, this graphite being highly objectionable because it considerably reduces machineability.

Casters are therefore prone to obtain the desired quality standard by maintaining a low silicon content and extending the annealing time, which, of course, is an expensive process.

Considerable researches and tests have been carried out with a view to reduce this costly annealing time and various methods have been proposed to this end: thus, the liquid smelt may be processed by overheating same, thus sterilizing the graphite germs; alternately, the smelt may be oxidized by adding suitable oxidizing adjuncts or using adequate atmospheres. These methods promote the production of a white-structure cast iron and therefore permit the use of iron compositions having a more graphitizing tendency. Low-content whiting substances such as bismuth, tellurium, cerium, may be added into the casting ladle. These methods are adequate and usually completed by adding elements such as bore promoting the decomposition of cementite without developing primary graphite. Thus, by adhering to strict percentages of these various additions the silicon content may be increased up to 1.7 or 1.8%, but beyond this content the risk of producing primary graphite increases and precludes any reliability as to the regularity of the iron quality or grade.

It is the essential object of the present invention to provide a method of manufacturing pearlitic and/or ferritic malleable cast iron, which is whitish and graphite-free when cast, and contains cementite even after the casting operation, the castings obtained from this malleable cast iron being subsequently annealed at a temperature and during a time sufficient to cause the decomposition of said cementite. According to this invention, on the one hand a cast iron is used of which the silicon content is such that it still represents at least 1.9% after ooling, and on the other hand mischmetall is added before the casting operation, the quantity of mischmetall thus added being suflicient to prevent said silicon content from causing lamellar graphite to develop in the casting during the setting thereof. Moreover, the sulfur content is kept to a percentage low enough to prevent said mischmetall, in the presence of said sulfur, from promoting the formation of lamellar graphite.

The cast iron will be so selected that after the casting and annealing operations malleable castings consisting principally of iron and having the following percentages by weight: C=2.3% to 2.7%; Si=1.9% to 2.30%; Mn 0.40%; Sg0.025% and Pg0.050% will be obtained. The annealing temperature ranges from 900 to 1,050 C.

So far as the applicant is aware, the high-silicon malleable cast iron according to this invention should contain a considerable percentage of lamellar graphite. Now with the addition of mischmetall by reason of 0.1% to 0.3%, provided on the other hand that certain requirements, such as the sulfur content of the cast iron, are duly met, the whitening effect is such that no trace of primary graphite is found, even in stock having a relatively high cooling modulus, such as 60-mrn. or 2%" round castings. As already known to those conversant with the art, mischmetall is a mixture of high-cerium rare earth. Although the cerium content is about 50% as a rule, the proportions of the other components are variable. Thus, for instance, mischmetall may have the following compositions, by weight:

(a) Ce:50%; La=25% and other lanthanides=25%.

(b) Ce=50%; La=40%; Fe=7%, and other lanthanides=3%.

(c) Cerium:50%; La:25%; Nd=14%; Pr=6%, and

other lanthanides=5%.

An annealing time of less than 1 hour at 950 C. may be sufficient for completely decomposing the cementite; a maximum time of 2 hours gives the certainty of obtaining this decomposition under the most unfavorable conditions. After cooling in still air the castings show graphite nodules in a pearlitic structure. In addition, maintaining for four-hours at 700 C. provides a ferritic structure; therefore, the duration of a total ferritizing cycle is reduced to less than 10 hours.

For carrying out the method of this invention the following measures may advantageously be taken:

The smelting charge may advantageously consist of sheet-metal press scraps and malleable iron rejects, in the proportion of 50% each. The carbon make-up consists of graphite, and silicon is given by adding ferro-silicon containing 75% of Si. The charge is melted in a highfrequency induction furnace having an acid, basic or neutral lining. After the smelting the charge is heated during minutes at 1,550 C., which is favorable to the absence of primary graphite. The iron is then poured into the ladle and then mischmetall in the proportion of 0.2% by weight is delivered into the casting jet. The scoria are allowed to settle, and the castings are subsequently formed at a temperature within the range of 1,420 C. to 1,430 C.

If the sulfur contents of the charges is abnormally high, desulfurizing is necessary and can be carried out by using any known and suitable process; thus, lime powder, calcium carbide powder, inter alia, may be used. The use of a basic-lined furnace promotes this desulfurization.

Thus, '60-mm. or 2% round castings have been obtained which had a whitish structure. By applying this method to a malleable iron for manufacturing balanceweights of truck crankshafts, brake-drum and hub assemblies, and differential cases for automobiles, sound rough castings having a clean skin and a white structure, without any primary graphite, have been obtained.

After annealing during 2 hours at 950 C. and cooling in a still atmosphere, the micrographic structure of a disk having a diameter of 160 mm. and a thickness of 25 mm. was as follows: 30 to 40 nodules per sq. mm., pearlite 90%, Brinnell hardness=269. The mechanical properties of this casting were tested on specimens according to the AFFOMA specifications. The following values were obtained in the pearlitic structure:

R (tensile strength) :70 H bar |(101,500 p.s.i.) E (apparent elastic limit)=54.5 H bar (79,050 p.s.i.) A% (total elongation)=6.

The analysis by weight of this casting gave the follOWing results: C=2.50%; Si=2.18%; Mn=0.22%; S=0.018%; P=0.011%, the balance=Fe.

In another smelt the casting assayed as follows: C=2.53%; Si=2.22%; Mn=0.25%; S=0.014%; P =0.015%, the balance=Fe.

A crankshaft balance weight having a Brinell hardness of 146 showed a fine annealing graphite on a ferrite background. The mechanical tests carried out on a AFFOMA specimen gave the following results:

R=43 H bar (62,350 p.s.i.) E=27 H bar (39,150 p.s.i.) A%:17.

The tests proved that to obtain satisfactory grades of malleable cast iron and to meet the requirements of this 4 invention, the cast iron composition must lie within the following limits by weight: C=2.3 to 2.7% Si=1.9 to 2.3% Mn 0.40% Ss0.025% P 0.050% the balance:Fe.

adding mischmetall to the casting ladle in an amount of 0.1% to 0.3% by weight of the melt; malleableizing annealing the casting produced between 900 C. and 1,050 C., during a maximum time period of two hours; and

cooling the annealed product in still air after malleableization to obtain a pearlitic structure.

2. A method as claimed in claim 1, wherein said cooling in still air is interrupted by maintaining the castings at a temperature of about 700 C. for about four hours to obtain a ferrite structure.

References Cited UNITED STATES PATENTS 1,591,598 7/1926 Williams -148--3 2,331,886 10/1943 Boegehold -1 148-3 2,370,225 2/1945 Boegehold 1483 2,501,059 3 /1950 Kluijtmans 148-35X 2,578,794 12/1951 Gagnebin 14835X 2,796,373 6/1957 Berg 14835X 2,901,386 8/1959 Saives 14835X 2,995,441 8/1961 Rubel 75-123 3,000,770 9/ 1961 Wittmoser 148-35 3,005,736 10/ 1961 Peras 14835 3,013,911 12/1961 Peras 14835 3,433,685 3/1969 Albertzart 148--35 3,445,299 5/1969 Laudenslager 148--138X 3,463,675 8/ 1969 Hunsaker 148-3 3,072,476 1/1963 Knapp 75129 3,155,498 11/1964 Jandras 75130 3,189,443 6/1965 Laudenslager 75--123 3,463,675 8/1969 Hunsaker 75123X L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R. 

